The simultaneous reception and transmission of electromagnetic signals within the same frequency band, with one aerial array for reception and one for transmission is customary, e.g. in central stations for mobile radiotelephony.
Attenuation between two coaxial aerials placed one above the other, operating with vertical polarization within the same frequency band and having average high directivity, has been experimentally determined to be: A.sub.dB =39+25 log(d/.lambda.), where d is the distance between the aerials and .lambda. is the wavelength in meters.
To obtain a required attentuation of 60 dB between two aerials, according to the aforementioned, without attenuation means between them consequently requires a mutual spacing of about 7 wavelengths.
This spacing between the aerials involves a problem in the design of masts and towers for aerial arrays, inter alia due to the wind resistance caused by the total structure. One method of decreasing this drawback is to introduce spacers and grounded screens between the aerials. Such arrangements are described, e.g. in DT No. 2354550 and corresponding U.S. Pat. No. 3,945,013 and DE No. 2629502 and corresponding U.S. Pat. No. 4,297,707.